The present invention relates to graphic plotters and, more particularly, in a graphics plotter having a motor for driving a penholding apparatus along an axis and a motor controller for accelerating and driving the motor to draw a series of vector lines, to the method for applying optimum acceleration to the motor comprising the steps of calculating the length of the next vector to be drawn; and, applying a step acceleration to the motor to draw short-length vectors less than a pre-established length and applying a ramp acceleration to the motor to draw vectors longer than said pre-established length.
A typical graphics plotter mechanism is shown in simplified form in FIG. 1. In the plotter 10, a pen 12 is gripped by a penholding mechanism 14 which is slidably mounted on a beam 16. The penholding mechanism 14 is moved along the beam 16 by a first motor 18 to create one axis of the two-dimensional plot on paper 20. Paper 20 is moved perpendicularly to the beam 16 under the penholding mechanism 14 by a pair of pinch-rollers 22 driven by a second motor 24 to create the other axis of the plot. Both motors 18, 24 are controlled by the control logic 26.
In the prior art, the motors 18, 24 have been accelerated according to one of two methods as preferred by the manufacturer of the plotter. FIG. 2 depicts a step acceleration curve. A momentary low level of power is applied to the motor as indicated at 28 to bring the moving parts of the plotter's mechanism up to "creep speed" (i.e., to overcome the starting inertia) after which maximum power is applied to the motor in a step or square wave manner as shown in the figure. Such step acceleration techniques apply a maximum shock to the components of the system and, therefore, because of costs of a design with the ability to withstand continued high G forces, typically acceleration is limited to lower levels to prevent long term damage to the components.
The second type of motor control typically applied in prior art plotters is a ramped acceleration as shown in FIG. 3. In this approach, the power is smoothly increased over time until at the maximum level. This smooth acceleration permits higher acceleration forces to be employed in a non-heavy duty, lower cost construction without damage to the plotter's components.
A velocity versus time graph of the two approaches is shown in overlay form in FIG. 4. The graph of FIG. 4 depicts, for example, the velocity of the penholding mechanism 14 when accelerated according to the two techniques of FIGS. 2 and 3. It should be remembered that the step acceleration is shown at lower G forces while the ramp acceleration is shown at higher G forces. As can be seen, initially the step acceleration increases in velocity faster than the ramp acceleration. After the cross-over point, however, the ramp acceleration curve moves more quickly to the terminal velocity than does the step acceleration curve. This has important ramifications which, heretofore, have gone unrecognized in the art. All so-called X-Y plotters plot the drawings that they create as a series of straight line vectors. The smoothness of apparently curved and angled lines is a function of the resolution of the plotter; that is, the smallest steps in which the motors 18, 24 can move. If the increments are very small, such as 0.001 inch, the lines, circles and curves all appear quite smooth. With larger steps of resolution (e.g. 0.005 inch), the vectors exhibit what is often referred to as "the jaggies"; that is, the intended curves and angled lines are visibly composed of a series of straight line steps. Basically, the vector lines fall into two classifications as indicated in FIGS. 5 and 6. In FIG. 5, we see two long vector lines 30. One extends between coordinate points A and B while the other extends between coordinate points B and C. The time to draw the long lines 30 is beyond the cross-over point on the velocity curves of FIG. 4. Thus, maximum through-put of the plotter can be achieved by employing the ramped acceleration of FIG. 3 on the motors 18, 24. By contrast, in FIG. 6, we see alpha-numeric text 32 wherein the letters of the text 32 are created by a series of very small vector lines 34 taking much less time to create than the time to reach the cross-over point in FIG. 4. Thus, maximum through-put can be achieved by employing the step acceleration of FIG. 2 to the motors 18, 24.
Additionally, it appears that while a high G step acceleration, in general, might be damaging to the mechanism if a given plotter over the long term, that same mechanism could withstand the same high G's of acceleration in the creation of small vector lines such as those employed to create text 32 without any long term detrimental effect to the plotter's mechanism. To do so, of course, would also greatly increase the through-put of the plotter in creating text.
Wherefore, it is the object of the present invention to provide a dual mode acceleration control for a plotter taking advantage of the benefits of both types of acceleration heretofore employed only independently by the prior art.
It is another object of the present invention to provide a new manner of motor control within graphics plotters employing the looking ahead at the characteristics of the next vector to be plotted and the modification of the control of the motor as a function of those characteristics.
Other objects and benefits of the present invention will become apparent from the description contained hereinafter when taken in conjunction with the accompanying drawing figures.